In an installation of the above type, it is necessary to provide at least one control device for controlling the flow of a stream of air through an air passage aperture. This air stream may, according to requirements, be cold air, heated air, or conditioned air. The air passage aperture may in particular be defined by a duct, a distribution vent, a channel, or other element of the installation.
From French patent publication No. FR 2 580 791A, it is known to make use of a control device of the type comprising a film which is wound on two rollers spaced apart from each other, the film being adapted to have an active or working position extending across the air passage aperture between the two rollers, this active portion being formed with open regions and solid or closed regions. The flow of the stream of air is selectively controlled according to the amount by which the film is wound on one roller relative to the other. Suitable driving means are provided for rotating the two rollers in synchronism with each other. The film is thus able to be displaced in one direction or the other by winding on to one of the two rollers, being simultaneously unwound from the other roller, thus offering various possibilities for control of the flow of the stream of air. Such a device is also referred to as a film obturator.
In a film obturator the film must be sufficiently thick, i.e. with a typical thickness of the order of a few tenths of a millimeter, in order to give enough mechanical strength to the film. In addition, due to the fact that the film must be put into various positions, each of which corresponds to a different setting in the control of the flow of the air stream, the strip of film must be sufficiently long, and its length may for example be anywhere in the range between 10 centimeters and 1 meter.
Thus, according to the relative position of the film on the rollers, there is likely to be, at any given time, a different number of turns of film on one roller than on the other. Since the film has a not insignificant thickness, and since the two rollers have the same diameter and rotate in synchronism with each other, the respective overall diameters of the rollers including the turns of film wound on them (this overall diameter also being referred to in this application as the "roll diameter"), vary continuously. This gives rise to consequent tensions which are exerted on the film and which can lead to rupture of the film.